Radio access technologies for cellular mobile networks are continuously being evolved to meet future demands for high data rates, improved coverage and improved capacity. Examples of recent evolutions of the wideband code-division multiple access (WCDMA) technology are the High-Speed Packet Access (HSPA) protocols. Currently, further evolutions of the third generation (3G) systems, 3G Long Term Evolution (LTE), including new access technologies and new architectures, are being developed within the 3rd Generation Partnership Project (3GPP) standardisation body.
A main objective of LTE systems is to provide a flexible access technology that can be used in existing frequency allocations and in new frequency allocations. Also, LTE systems should enable the use of different duplex solutions. For example, both frequency division duplex (FDD) and time division duplex (TDD), where the uplink and downlink are separated in frequency and in time, respectively, should be supported to provide usage in both paired and unpaired spectrum.
An access technology based on Orthogonal Frequency Division Multiplexing (OFDM) for the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) for the uplink, for example, allows such flexible spectrum solutions.
Since the LTE concept is being designed to support fast scheduling in frequency and time both for the uplink and the downlink, the resource assignment in time and frequency should be preferably adjustable to the users' momentary traffic demand and channel variations. In the LTE uplink it is possible to schedule several users in one Time Transmission Interval (TTI) by assigning different frequency segments to different users. To maintain the single carrier structure, each user should only receive contiguous assignments in frequency as illustrated in FIG. 1.
Referring now to FIG. 2, a scheduler 202 in an evolved Node B (base station) 204 may perform resource assignment. Scheduling resources among two or more users in the uplink is complicated by the fact that the scheduler 202 is not automatically aware of each user's uplink data and resource demand. That is, for example, the scheduler 202 may not be aware of how much data there is in the transmit buffers of each user's mobile terminal 206 (e.g., mobile phone, portable digital assistant, or any other mobile terminal). Mobile terminal 206 may also be referred to as user equipment (UE). In order to support fast scheduling, the scheduler 202 would have to be made aware of the UE's momentary traffic demands (e.g., the transmit buffer status).
The basic uplink scheduling concept is illustrated in FIG. 2. Typically, to inform the uplink (UL) scheduler 202 of the UE's momentary traffic demands, the system 200 supports (i) a dedicated scheduling request (SR) channel and (ii) buffer status reports. Alternatively, a synchronized random access channel (RACH) can be used for the same purpose.
The scheduler 202 monitors each UE's traffic demands and assigns resources accordingly. The scheduler 202 informs a UE (e.g., UE 206) of a scheduling decision by transmitting resource assignments 208 to the UE. In addition, there is a possibility to configure a UE to transmit channel sounding reference signals to enable the evolved Node B (eNodeB) to do broad band channel estimation for fast link adaptation and channel dependent scheduling.
A synchronized UE also has the opportunity to use, as a fallback solution, the Random Access Channel (RACH) to request a UL resource. In general, however, the RACH is intended mostly for non-synchronized UEs. In the dedicated SR channel approach, each active UE is assigned a dedicated channel for transmitting messages that indicate to the eNodeB that the UE requires a UL resource. Such a message is referred to as a scheduling request (SR) 210. The benefit with this method is that no UE identifier (ID) has to be transmitted, since the UE is identified by virtue of the “channel” it uses. Furthermore, in contrast to the contention based approach, no intra-cell collisions will occur.
In response to receiving an SR 210, the scheduler 202 may issue to the UE a scheduling grant (SG) 208. That is, the scheduler may select the resource(s) (e.g., time slot and/or frequency) the UE shall use and communicate this information to the UE. The scheduler 202 may also select, with support from the link adaptation function, a transport block size, a modulation scheme, coding scheme and an antenna scheme (i.e., the link adaptation is performed in the eNodeB and the selected transport format is signalled together with information on the user ID to the UE). The scheduling grant addresses a UE and not a specific radio bearer. In its simplest form, the scheduling grant is valid only for the next UL TTI. However, to reduce the amount of control signalling required, several proposals with alternative durations are possible.
After transmitting an initial SR, the UE may transmit a more detailed buffer status report to the scheduler 202. The buffer status report may be transmitted in-band (e.g., the buffer status report may be included as part of a medium access control (MAC) header). It is a common view in, for example, 3GPP that the buffer status report should contain more information than is contained in the initial SR.
The above described procedure is further illustrated in FIG. 3. As shown in FIG. 3, a UE 302 having data to transmit to an eNodeB 304 first transmits an SR 306 to the eNodeB 304, which SR 306 is then processed by an uplink scheduler 308 of eNodeB 304. In response to SR 306, uplink scheduler 308 transmits an SG (e.g., resource assignments) 310 to UE 302. Thereafter, UE 302 transmits data 312 to eNodeB 304 together with a buffer status report 314, which report is processed by the uplink scheduler 308. As discussed above, buffer status report 314 may be transmitted in-band with data 312.